This invention relates to a starting circuit and a starting method for an SCR inverter-motor system, where the SCRs in the inverter are motor commutated during normal running operation.
One form of a variable speed drive consists of a synchronous motor driven by a motor-commutated SCR inverter. The SCRs (silicon controlled rectifiers) in the inverter are gated or turned on in predetermined pairs or sets and in a prescribed sequence in order to convert an applied d-c bus voltage, received over a d-c bus or link from a controlled d-c power supply, to a-c voltage for application to the motor. During normal operation, the frequency of the inverter (namely, the gating of the SCRs) is synchronized with the frequency or speed of the motor, usually with rotor position sensors or by sensing the motor voltage or flux. Before each pair of SCRs are gated into conduction, at least one previously conducting SCR must be commutated or switched off so no current flows therethrough. When the motor is of the synchronous type motor commutation may be employed, the motor terminal voltage being used to commutate the inverter SCRs. This obviates the need for forced commutation circuitry. As is well understood in the art, the inverter-motor system may be constructed and operated to present a leading power factor to the inverter a-c terminals, the alternating current in each of the motor's stator windings thereby always leading the alternating motor voltage across that winding. Basically, it involves providing a motor back EMF (electromotive force) that is greater than the applied inverter voltage. The back EMF is induced in the stator windings by the rotating flux produced by the magnet (either a permanent magnet or an electromagnet) in the rotor. With a leading power factor, when an inverter SCR is gated on it will cause the back EMF to reverse bias and to turn off a previously conducting SCR, the motor current thereby effectively transferring to the on-coming SCR.
The problem presented with a motor-commutated SCR inverter is that the motor must be running in order for the rotating flux from the rotor to cut the stator windings and induce therein a back EMF of adequate magnitude to commutate the SCRs. Starting apparatus of some type must therefore be employed to start the motor rotating and bring it up to a speed at which the required back EMF will develop and take over the commutation of the inverter SCRs. Starting systems have been developed which regulate the d-c power supply and the sequential gating of the inverter SCRs to effectively apply time-separated current pulses to the stator windings to effect step-by-step motor rotation at an increasing or accelerating rate until the back EMF reaches a threshold level at which motor commutating and normal running operation occur. During starting the inverter SCRs may be forced commutated, before a new pair of SCRs are gated on, by reducing the d-c link or bus current to zero. In these prior starting systems, various arrangements are used to synchronize the gating or firing of the inverter SCRs to the motor, namely to the rotor position or back EMF. For example, the desired synchronization may be achieved by a position sensor mounted on the motor shaft. When a position sensor is not used, sophisticated and complex computation circuits may be employed to obtain synchronizing information, for controlling the inverter operation, from the motor terminal voltage. In accordance with another prior starting technique, referred to as the "open loop" method, the inverter frequency is slowly increased up to the motor commutation speed and the system is switched over to motor commutating. If the inverter and motor are properly synchronized, the motor will run normally. On the other hand, if a commutation failure occurs the system is stopped and restarted. This trial and error procedure is repeated until normal motor commutation takes place.
The starting system of the present invention constitutes a significant improvement over these prior starting schemes. Highly reliable, fast and smooth starting is achieved by means of a relatively simple and inexpensive circuit arrangement, requiring no shaft position sensors or sophisticated computation circuits and not employing trial and error techniques.